1. Field of the Invention
The present invention relates to a hydraulic braking system for motor vehicles and, more particularly, to an antiskid braking system wherein hydraulic pressure applied to wheel cylinders is controlled in response to road surface conditions and vehicle conditions in such a manner that the vehicle is stopped without skidding and at a reduced braking distance while maintaining good steerability of the vehicle.
2. Description of the Related Art
A known antiskid braking system comprises a solenoid operated shutoff valve in a main hydraulic conduit connecting a master cylinder to respective wheel cylinders, the wheel cylinders being, in turn, connected to a reservoir through a three-position solenoid valve, as described later in more detail. The system further includes a hydraulic pump which serves to pump the brake fluid in the reservoir back to the wheel cylinders, the hydraulic pump being designed to be driven by an electric motor coupled thereto. This known system operates as follows. When the brake is applied and a sensing means senses that the brake is about to lock any one of the vehicle wheels, the shutoff valve is closed to interrupt hydraulic pressure being applied by the master cylinder to the wheel cylinders, and simultaneously therewith, the three-position valve is shifted for a predetermined short time interval to a position in which the wheel cylinders are connected to the reservoir so that a small amount of brake fluid in the wheel cylinders is released toward the reservoir, causing the hydraulic pressure in the wheel cylinders to be relieved and thereby reducing the braking effort. This cycle of operation is repeated at short intervals until the tendency for the wheel to lock disappears, whereupon the three-position valve is then moved to another position in which the brake fluid pumped by the hydraulic pump is fed back to the wheel cylinders, thereby gradually increasing the hydraulic pressure therein and the braking force.
One of the disadvantages of this known antiskid braking system is that it uses an electric motor to drive the hydraulic pump. Since the hydraulic pump requires a large amount of power, it has been necessary to use an electric motor having relatively large size and weight.
The use of electric motor has given rise to another problem. Since by its very nature, the electric motor is unsuitable for being driven intermittently at very short time intervals, it has been necessary to continuously energize the motor throughout a braking period of the vehicle, which, in turn, entailed that the hydraulic pump be continuously operated. This causes "kick back" of the brake pedal due to the pumping of the brake fluid back to the master cylinder, as explained later in more detail.
The above-mentioned braking system also uses a three-position solenoid valve, such as that described in Japanese Utility Model publication No. 58-17169, for controlling the hydraulic pressure at the wheel cylinders. It has been recognized that this solenoid valve is disadvantageous in that it is large in size and complex in structure, because it has three positions; a first or normal position, a second or intermediate position, and a third or extreme position. Furthermore, when the valve is to be held in the intermediate position, the valve must be energized by a control signal having an intermediate intensity. This requires the supply current to be limited thereby limiting the responsiveness of the solenoid valve, and hence, of the braking system.